Housings equipped with viewing windows are applied in, among other fields of use, industrial measurements technology. Forming an example of this are measuring devices, in the case of which there is provided in a housing an on-site display, especially for display of measured values and/or operating parameters, which are externally perceivable through the viewing window.
Placed on housings with viewing windows, especially for their use in industrial measurements technology, are a number of requirements, depending on location of use. Among these are especially hygienic requirements for applications in the pharmaceuticals and foods industries.
Already in use today are viewing window equipped housings, which have an outer wall, in which, as a rule, a circular opening is provided, which is in communication with the interior of the housing. Used for the viewing window pane is usually a planar, as a rule, circular disk shaped, piece of glass or plastic, whose area is greater than the area of the opening. This disk is secured in the interior of the housing in such a manner that an outer edge of the disk lies on an inner side of the outer wall surrounding the opening.
These housings have the advantage that the outer wall of the housing provides a mechanically stable bearing surface for the viewing window with reference to pressures occurring in the housing interior.
Disadvantageous is that, in the outwardly located part of the opening before the pane, water or fouling from the environment of the housing can collect. In this way, with time, deposits can form on the viewing window.
The opening forms a hollow space, which is difficult to keep clean. Correspondingly, these housings are not suitable for applications with high hygienic requirements.
Moreover, deposits in the opening can lead to a degrading of the visibility of the housing interior through the viewing window.
Published international application, WO 2012/000526 A1 describes a housing equipped with a viewing window, in the case of which the viewing window contains a planar, plastic pane, which is mounted flushly in an opening in the housing. For this, an outer edge of the disk shaped viewing window pane lies with interpositioning of a seal on an outwardly facing surface of a ledge surrounding the opening in the outer wall. The disk-shaped viewing window includes an outer lateral surface conically tapering in in the direction facing out of the housing, and is mechanically secured on the surface of a ledge by means of forming of a bead of the material of the housing surrounding the opening and the surface of a ledge in order to produce a shape interlocking connection in such a manner that the resulting joint between viewing window and housing on the outside of the housing forms an essentially jointless, step free transition.
The forming of the bead represents an additional, technically demanding, working step, in the case of which the applied plastic pane is, in given cases, mechanically strongly loaded. Consequently, only plastic materials can be applied, which can withstand such loading.
In contrast, industrial applications regularly require use of a viewing window pane of a material, which due to its chemical and mechanical properties is optimally suitable for the particular application. In such case, depending on application, different chemical properties, such as e.g. chemical durability for exposure to acids, bases or aggressive cleaning agents, and physical properties, such as e.g. temperature resistance, scratch resistance or a smooth surface, are required.